A process for quantifying brain function may involve analysing the spontaneous or stimulus locked scalp recordable electrical activity from a subject. For example, this includes analysing the waveform of early, middle and/or late stimulus evoked components (e.g. as described in International Patent Publication WO 2001/74248); or spectral analysis of spontaneously recorded activity (not in response to a particular or general stimulus) using frequency or time domain methods (e.g. as described in European Patent Application EP0898234); or a hybrid approach in which both spontaneous and evoked EEG activity is analysed to determine brain state (e.g. as described in International Patent Publication WO 2004/054441).
While such methods have been shown to have clinical efficacy when appropriately constructed statistical discriminant functions are employed, it is unclear what physiological aspects of behaviour and brain function such measures reflect. For instance, these approaches may be detecting changes in EMG activity, and not EEG activity. The Messner report (published in Anesth Analg, 2003, 97, pp. 488-491) describes how the bispectral index declines during neuromuscular blockade in fully awake persons. Recent theoretical and experimental work by Liley et al (as described in International Patent Publication WO 2004/064633) proposes a specific theoretical framework that enables the construction of more physiologically specific measures of brain function. This framework enables greater structural and functional specificity in representing changes in cortical activity induced by a variety of internal and external factors.
In assessing a subject's brain function such as, for instance, during health, disease and/or therapeutic intervention, or for the purpose of controlling or interfacing with some external hardware device such as a computer when used for control or entertainment purposes, it is important to distinguish the factors that give rise to changes in brain function. For example, this includes changes in brain (cortical) state (i.e. the brain's inherent receptivity to input from subcortical or distant cortical sources) and changes in the level of cortical neuronal input (which may occur as a consequence of altered input to the cerebral cortex). While an analysis of the early components of a variety of event related potentials (ERP) may provide information regarding the integrity of the various input pathways to the cortex, this technique is inherently limited as not all cortical areas are the recipient of peripherally derived sensory information. For example, the frontal cortex neither directly nor indirectly (through subcortical nuclei) receives any sensory information. Another limitation of this approach is that, in order to obtain a sufficient signal-to-noise ratio, the evoked response of a number of sequentially presented stimuli must be determined, which clearly limits the temporal resolution of the results obtained. However, there are methods that attempt to improve the temporal resolution by using some form of forecasting method (e.g. as described in International Patent Publication WO2001/74248).
Quantitative EEG (QEEG) methods involving spectral analysis using time or frequency domain methods (e.g. as described in European Patent Application EP0898234) are unable to distinguish between changes in cortical input and brain (cortical) state, because such techniques are unable to make assumptions regarding the physiological sources of changes in EEG spectral power. This is principally a consequence of the heuristic approach of current QEEG methods.
Accordingly, it is difficult to determine whether changes in EEG signals from a subject are caused by changes in cortical input (e.g. to different areas of the brain), or are a consequence of qualitative and quantitative changes in how the cortex responds to this input. There is also no satisfactory way for representing changes in a subject's brain function based on changes in cortical responsive state and the level of cortical neuronal input.
It is desired to address one or more of the above, or to provide at least a useful alternative.